The present invention generally relates to manufacturing aircraft components, such as skin structures formed by a skin panel reinforced on its inner face by a plurality of stringers structurally bonded to the skin panel.
In manufacturing components for the aeronautical industry, especially for manufacturing structural elements of an aircraft, the use of composite materials formed by an organic matrix and continuous fibers, for example carbon fiber-reinforced plastic (CFRP), oriented in one direction in one and the same ply is well known. United States patent application US 2010/0233424 A1 is an example of these techniques.
An aircraft skin structure, for example part of the fuselage or part of a wing, is conventionally formed by a skin panel and by a series of stringers attached to one of the faces of the panel by means of co-bonding in order to reinforce it. The stringers are arranged in a longitudinal direction with respect to said panel, and a series of transversely arranged ribs collaborating with the stringers to provide strength to the panels and to improve the stability under compression or shearing thereof are further incorporated.
These composite materials are used both for manufacturing skin panels and for manufacturing the stringers, which can be manufactured with different sections, for example with a T-shaped section, I-shaped section, L-shaped section, trapezoidal-shaped section, etc.
The drawbacks existing with stringer run-outs is well known in the aeronautical industry because a redistribution in the load transfer between the stringer and the panel to which it is attached occurs at those run-outs, bringing about a concentration of stresses (tangential in the plane of the attachment and of peeling outside the plane) in the bonded attachment in that zone, which can cause the stringer and panel to become detached.
Stringer stiffness in skins is obtained mainly as a result of the plies with a fiber orientation at 0°, i.e., with the fibers oriented in the same longitudinal direction of the stringer. However, this conventional arrangement with most plies at 0°, extending along the length of the stringer, entails a limitation when reducing the load supported by the run-outs, because its stiffness must be reduced at the run-outs precisely to reduce the load which they support and must transfer to the skin through the bonded attachment.
The solutions known in the state of the art for overcoming the problems associated with the load transfer at stringer run-outs are generally based on reducing the stringer cross-sectional area, usually by means of reducing the stringer height or, additionally, by means of progressively reducing the number of plies towards the run-out. With these techniques the thickness, and therefore the stiffness of the stringer run-out, is reduced (the elastic modulus is reduced), encouraging an early start of the progressive redistribution of loads between the stringer and the panel at the same time.
An example of these solutions is described in United States patent applications US 2005/0211846 A1 and US 2012/0100343 A1.
However, these conventional solutions have certain drawbacks because, for example, with the solution of reducing the stringer height, stringer efficiency is reduced at the same time to increase the stability of the panel preceding the stringer run-out.
In the case of the second solution referring to the reduction of plies, it has limitations concerning the lowest possible number of fabrics at the stringer run-out below which manufacture and industrialization defects arise during compaction and cobonding of the stringer to the panel.
This second drawback limits the complete elimination of all the fabrics at 0° at the stringer run-out and therefore maintains a high elastic modulus therein with respect to a solution that would allow completely eliminating or having fewer fabrics at 0° at the run-out thereof.
Other known solutions are based on using several types of plies with a different elastic modulus at the run-out, i.e., with different properties, for example using fibers of different materials providing less stiffness. However, although the use of different types of plies or sheets is feasible, it is not contemplated by the methods in use for calculating structures such as those contemplated herein, in addition to excessively complicating the manufacturing process of these components, so it is not widely used today.